Optical disc drives have been widely developed in recent years as a means for recording and reproducing large amounts of data. Various approaches have been used to achieve ever higher recording densities, and one of these is the phase-change optical disc drive using a reversible state change between crystalline and amorphous phases.
A phase-change optical disc drive forms marks (amorphous parts) and spaces (crystalline parts) disposed between marks on an optical disc medium by emitting a semiconductor laser to the optical disc medium at two power levels, a peak power level for making a crystalline part amorphous, and a bias power level for making an amorphous part crystalline.
Because reflectivity differs in marks and spaces, a recorded signal is read during playback by using this difference in reflectivity.
The configuration of a conventional phase-change optical disc drive is shown in FIG. 14. In FIG. 14, reference numeral 1001 denotes an optical disc, reference numeral 1002 denotes an optical head, reference numeral 1003 denotes a playback means, reference numeral 1004 denotes a playback signal quality detection means, reference numeral 1005 denotes an optimum recording power determining means, reference numeral 1006 denotes a recording means, reference numeral 1007 denotes a laser drive circuit, and reference numeral 1008 denotes a recording power setting means.
The track configuration of a conventional optical disc 1001 is shown in FIG. 15. The optical disc 1001 is an optical disc having recording areas in both the groove-shaped tracks (groove tracks 1101) and the tracks between grooves (land tracks 1102), forming a continuous spiral by alternating the groove tracks and land tracks every revolution.
After the optical disc 1001 is loaded into the optical disc drive and specific operations for identifying the disc type and rotation control are completed, the optical head 1002 moves to an area for determining the optimum emission power. Note that this area is provided at the innermost circumference or the outermost circumference of the disc, and is a recording area that is separate from the user area for recording user data.
Both peak power and bias power are determined with a phase-change optical disc drive, and the method for determining the peak power is described here.
First, initial values for the peak power and bias power are set in the laser drive circuit 1007 by the recording power setting means 1008. The power for recording to a land track and the power for recording to a groove track are equal at this time.
A signal for recording one land track revolution and one groove track revolution from a specific position is then sent from the recording means 1006 to the laser drive circuit 1007 and recorded by the optical head 1002. The light output of the semiconductor laser that is a component of the optical head 1002 is gathered as a light spot on the optical disc 1001, and a recording mark is formed according to the light emission waveform.
When land track and groove track recording ends, the semiconductor laser of the optical head 1002 emits at the playback power level, the track just recorded is played back, and a signal 1009 that varies according to the presence of a recording mark on the optical disc 1001 is input to the playback means 1003 as a playback signal. The playback signal 1009 is subjected to playback signal processing such as amplification, waveform equalization, and digitizing by the playback means 1003, and a signal 1010 is then input to the playback signal quality detection means 1004.
The playback signal quality detection means 1004 detects the signal quality of the signal 1010, and inputs the detection result to the optimum recording power determining means 1005.
The playback signal quality detection means 1004 here detects the BER (byte error rate) when the recorded signal is played back. The BER detected at this time is the average for the reproduced track. The relationship between peak power and BER is shown in FIG. 16.
The horizontal axis is peak power and the vertical axis is the BER in FIG. 16. If the playback conditions are equal, recording accuracy generally increases as the BER decreases. Therefore, if the BER is less than or equal to a given threshold value, the detection result is OK, and if the BER is greater than or equal to the threshold value, the detection result is NG.
Following the flow chart in FIG. 17, for example, the optimum recording power determining means 1005 sets the peak power higher than the initial power level if the first result from the playback signal quality detection means 1004 is NG, for example, sets the peak power lower than the initial power level if the result is OK, and then records and plays a land track and groove track at the set peak power in the same way as before.
If the first result from the playback signal quality detection means 1004 is NG and the second result is OK, the optimum recording power determining means 1005 determines the average power of the present peak power and the previous peak power plus a specific margin to be the optimum recording power.
If the first result from the playback signal quality detection means 1004 is OK and the second result is NG, the optimum recording power determining means 1005 determines the average power of the present peak power and the previous peak power plus a specific margin to be the optimum recording power.
However, because the area for determining the optimum emission power and the user area for recording user data are in separate places, a relative tilt can occur between the two areas due, for example, to disc warpage or how the head is mounted, and there are cases with the prior art described above where the user data is recorded with power that is effectively weaker than the emission power set in the area for determining the optimum emission power, and conversely, cases where the user data is recorded with power that is effectively stronger than the emission power set in the area for determining the optimum emission power.
As shown in FIG. 13, mark width is generally fatter (thicker) when recorded with strong emission power. Therefore, a problem is that when a track which is recorded with an effectively strong emission power using one recording device is then overwritten with an effectively weaker emission power using another recording device, an unerased mark remnant results in the area where the mark was formed on the base layer. This unerased mark remnant becomes noise during playback, and playback performance drops as a result.
The present invention is directed to the above problem, and provides an optical disc drive and an optimum recording power determination method with an object of recording correctly even when the effective emission power varies.